This invention relates to display devices and displays and, in particular, to a passive or active light valve or shutter for use in displays. Electric display devices convert electric signals transmitted from electric or electronic devices into optical signals that can be recognized by humans. The optical signal is displayed as an image in the form of digits, characters, or graphics. Electric displays are also divided into active or passive types. When the optical signal is displayed by light emission, the display is termed an active display, whereas when the display works by modulating incident light through reflection, scattering, interference, or similar phenomena, using an external light source it is termed a passive display.
Displays may be further subdivided into several further categories, as follows:
LCD liquid crystal display PA1 ECD electrochemical display PA1 EPID electrophoretic image display PA1 SPD suspended particle display PA1 TBD twisting ball display PA1 PLZT transparent ceramics display; and PA1 ELVD electrostatic light valve display.
An ELVD is described in U.S. Pat. No. 3,989,357 issued Nov. 2, 1976 to Charles G. Kalt. Kalt's ELVD is a passive device and consists of a fixed electrode to which a movable coiled resilient sheet electrode is attached with an insulating layer separating the two electrodes. The coiled electrode is caused to unroll upon application of an electric potential between the two electrodes. The coiled electrode thus acts as a light shutter.
The inner surface of the coiled electrode has a color or reflectivity that is different from that of the fixed electrode, in which case, the device to an observer changes hue or reflectivity when a potential is applied. Alternatively, the fixed electrode may be transparent to a selected portion of the electromagnetic spectrum and the coiled electrode may be opaque. In this case, the device is said to operate in the transmissive mode and a light source positioned behind the device would transmit light when no potential is applied and would not transmit light when a potential is applied.
Other patents pertaining to ELVD's are U.S. Pat. Nos. 3,897,997; 4,094,590; 4,235,522 and 4,248,501.
The above patents generally teach the use of metallized plastic sheets as the movable or coiled element. These sheets are formed into a roll using heat and a mandrel or by bonding two plastic sheets, one of which is prestressed in one direction before bonding. There are a number of problems associated with these approaches when considering manufacturing cost, reliability, temperature effects and electrical charge control in these devices. The methods described require individual handling of each shutter to form the roll and bond it to the fixed electrode. Some of the problems of handling are described in U.S. Pat. No. 4,094,590, which describes the formation of wrinkles. These prior art devices rely on the elastic properties of plastic, which is undesirable, since these properties can vary widely with temperature and humidity and often change as they age. The flexing characteristics of the movable electrode are determined by these elastic properties, therefore the threshold voltages are likely to drift. U.S. Pat. Nos. 4,235,522 and 4,248,501 describe some of the issues of charge control in the insulator. These problems are more severe than is indicated in the patents. Even small amounts of accumulation or drift of charge in the plastic materials described will cause large amounts of threshold drift in the light valves, which is undesirable for many applications.